1. Field of the Invention
The present invention relates to low-pass filtering technology, and more particularly, to a high-order low-pass filtering circuit and method characterized by reduced noise and physical size.
2. Description of the Related Art
Generally, wireless receivers (e.g., direct conversion receivers) or low intermediate frequency receivers use a low-pass filtering circuit to attenuate a blocker signal apparent in the output signal of a frequency conversion mixer. Figure (FIG.) 1A illustrates a conventional first-order low-pass filtering circuit. FIG. 1B is a graph illustrating the frequency pass characteristics for the first-order low-pass filtering circuit illustrated in FIG. 1A. Referring to FIG. 1A, the conventional first-order low-pass filtering circuit includes a resistor R1 and a capacitor C1. A cut-off frequency (fc) of the conventional first-order low-pass filtering circuit illustrated in FIG. 1A is may be expressed by Equation (1):fc=1/(R1C1)  (1)
Thus, the cut-off frequency of the conventional first-order low-pass filtering circuit is determined by the resistance value of resistor R1 and the capacitance of capacitor C1. The resistance value of resistor R1 and the capacitance of capacitor C1 need to be very large in order to set the cut-off frequency fc to several hundreds of Hz.
However, in order to set the resistance value of resistor R1 and the capacitance of capacitor C1 to relatively large values, the layout area of resistor R1 and capacitor C1 must normally be increased using conventionally understood on-chip processes. However, there are practical limits to the amount by which the capacitance of capacitor C1 may be increased using on-chip processes. Thus, an external capacitor is often used to implement capacitor C1, but this approach typically requires that even more space be allocated to the low-pass filter.
In order to address this disadvantage, a low-pass filtering circuit including an amplifier 10, as illustrated in FIG. 2A, has been suggested. The low-pass filtering circuit including amplifier 10 decreases the cut-off frequency by feeding-back an output signal from amplifier 10 to capacitor C1. FIG. 2B is a graph illustrating the frequency pass characteristics of the low-pass filtering circuit illustrated in FIG. 2A.
Referring to FIGS. 2A and 2B, the cut-off frequency of the low-pass filtering circuit illustrated in FIG. 2A is lower than that of the first-order low-pass filtering circuit illustrated in FIG. 1A by an amount, “ΔF”. However, the roll-off slope of the pass characteristics for the first-order low-pass filtering circuit illustrated in FIG. 1A is about the same as that of the low-pass filtering circuit illustrated in FIG. 2A. Accordingly, more ideal filtering results cannot be achieved. And while the roll-off slope characteristics may be improved by the use of a high-order low-pass filtering circuit, such circuits tend to increase noise.